An electroconductive polymer having an acidic group like a sulfonic acid group and/or a carboxylic acid group (carboxy group) exhibit excellent solubility in water or an organic solvent due to the hydrophilic property of the acidic group, and thus studies are made on various synthetic methods.
Furthermore, an electrical conductor (electroconductive polymer film) having the electroconductive polymer with an acidic group as a main component, a laminate provided with the electrical conductor, and a method for manufacturing them are reported (see, Patent Document 1, for example).
However, while the electroconductive polymer having an acidic group has excellent solubility in a water soluble solvent, the electrical conductor formed of an electroconductive composition containing the polymer has insufficient water resistance. For such reasons, the electrical conductor has limitation in terms of use, that is, it is not suitable for a use in which water resistance is required.
Furthermore, for forming an electrical conductor, an electroconductive composition is generally applied on a substrate, several times by an impregnation method or the like, and plural coating films are overlaid to form an electrical conductor with desired thickness. In this regard, since the electrical conductor has insufficient water resistance, an already-formed coating film is dissolved in an electroconductive composition which is applied onto it. Thus, there is a case in which it is difficult to achieve overlapping application.
In order to solve the aforementioned problems, a method including forming a coating film on a substrate by applying an electroconductive composition containing an electroconductive polymer with an acidic group and heating the electrical conductor at 150 to 280° C. is suggested (see, Patent Document 2, for example).
According to this method, as the electrical conductor is heated at pre-determined temperature, the acidic group is appropriately dissociated, and as a result, water resistance of the electrical conductor is improved.
However, when a coating film is heated as it is described in Patent Document 2, the acidic group is intentionally dissociated to improve the water resistance of an electrical conductor so that the electroconducting property is easily lowered.
A method of adding a specific basic compound to suppress the dissociation of an acidic group is also suggested (see, Patent Document 3, for example).
According to this method, a lowered electroconducting property can be suppressed but, as the dissociation of the acidic group is suppressed, it was unable to have the water resistance.
Meanwhile, a solid electrolyte condenser having a solid electrolyte layer, which is formed, on top of a dielectric layer (dielectric oxide film) formed on a surface of a positive electrode (coating film forming metal) consisting of a porous body of a metal with valve action (valve metal) like aluminum, niobium, tantalum, titan, and magnesium, by using an electroconductive polymer as a solid electrolyte, and a negative electrode in order has been developed.
This solid electrolyte condenser has conductivity of a solid electrolyte which is 10 to 100 times higher than that of a solid electrolyte condenser of the related art in which manganese dioxide is used as a solid electrolyte, and as ESR (equivalent series resistance) can be greatly reduced, it is expected to be used for various applications including absorbing high frequency noise of a small electronic device.
According to a recent tendency of having high frequency and large current for an integrated circuit, a solid electrolyte condenser having low ESR, high capacitance, and high reliability is required.
As a monomer for constituting an electroconductive polymer, pyrrole, thiophene, 3,4-ethylene dioxythiophene, aniline, and the like are known.
Furthermore, as a method for forming a solid electrolyte layer on a dielectric layer, a chemical oxidative polymerization or an electrolytic polymerization is generally used.
However, as a polymerization reaction occurs on a dielectric layer, the chemical oxidative polymerization and an electrolytic polymerization easily allow an incorporation of impurities in a solid electrolyte layer, providing a cause for having short circuit.
In addition, it is easy to have a complicated manufacturing process.
As a measure for solving this problem, a method of forming a solid electrolyte layer without performing a chemical oxidative polymerization or an electrolytic polymerization on a dielectric layer, specifically, a polymer suspension coating method is known.
The polymer suspension coating method is a method for forming a solid electrolyte layer in which a monomer is polymerized in advance to give an electroconductive polymer and a dispersion containing the electroconductive polymer is coated and dried on a dielectric layer to form a coating film.
However, in case of the polymer suspension coating method, it is difficult for a dispersion of an electroconductive polymer to impregnate in the inside of a dielectric layer. As a result, a solid electrolyte layer is not easily formed in the inside of the fine irregularities (pores) of a dielectric layer and it is formed only on a surface layer. Thus, there is a problem that the electric capacitance exhibition rate of the obtained solid electrolyte condenser is low.
In this regard, a method of forming a solid electrolyte layer by impregnating a dielectric layer in an electroconductive polymer, which is soluble in water or an organic solvent, is suggested (see, Patent Documents 4 and 5, for example).
According to this method, it is possible to impregnate an electroconductive polymer even in the inside of a dielectric layer so that a solid electrolyte can be also formed in the inside of the fine irregularities (pores) of a dielectric layer.
In particular, by adding a basic compound to a solution of an electroconductive polymer, deterioration caused by heat which is generated during manufacturing of a condenser can be prevented, and thus the electric capacitance exhibition rate of a solid electrolyte condenser can be further increased.
In recent years, however, a positive electrode consisting of a porous body of a valve metal is micronized in accordance with a tendency of having a solid electrolyte condenser with small size⋅light weight⋅large capacitance, and therefore it has fine pores with various shapes.
Thus, the inside of a dielectric layer formed on a surface of such positive electrode also becomes micronized and complicated. Accordingly, sufficient impregnation of fine pores with an electroconductive polymer is also required.
Furthermore, in case of a solid electrolyte condenser obtained by the method described in Patent Documents 4 and 5, there can be a case of having lowered electric capacitance when a durability test like heating at a temperature higher than the temperature applied during condenser manufacturing process is performed.